A lithographic process includes the patterned exposure of a resist so that portions of the resist can be selectively removed to expose underlying areas for selective processing such as by etching, material deposition, implantation, and the like. Traditional lithographic processes utilize electromagnetic energy in the form of ultraviolet light for selective exposure of the resist. As process nodes continue to shrink, optical lithography (or photolithography) has become increasingly inadequate because of its diffraction limit. Wavelength reduction, mask and illumination optimization, numerical aperture increase, and proximity correction can only improve resolution to a certain extent. The semiconductor industry is seeking alternatives to optical lithography. Charged particle beams have been used for high resolution lithographic resist exposure. In particular, electron beams have been used because the low mass of electrons allows relatively accurate control of an electron beam at relatively low power. Electron beam lithographic systems may be categorized as electron beam direct write (EBDW) lithography systems and electron beam projection lithography systems.
In EBDW lithography, the substrate is sequentially exposed by means of a focused electron beam, wherein the beam either scans in the form of lines over the whole specimen and the desired structure is written on the object by corresponding blanking of the beam, or, as in a vector scan method, the focused electron beam is guided over the regions to be exposed. The beam spot may be shaped by a diaphragm. EBDW is distinguished by high flexibility, since the circuit geometries are stored in the computer and can be easily varied. Furthermore, high resolutions can be attained by electron beam writing, since electron foci with small diameters may be attained with electron-optical imaging systems. However, the process is time-consuming due to the sequential, point-wise writing. Therefore, EBDW is typically used for the production of the masks required in projection lithography. Even for patterning photomasks, which generally have dimensions 4× those of the wafer, electron beam lithography takes multiple hours to write an entire mask.
In electron beam projection lithography, analogous to optical lithography, a larger portion of a mask is illuminated simultaneously and is imaged on a reduced scale on a wafer by means of projection optics. Since a whole field is imaged simultaneously in electron beam projection lithography, the attainable throughputs can be markedly higher in comparison with electron beam writers. However, in a conventional electron beam projection lithography system, a corresponding mask is necessary for each structure to be exposed. The preparation of customer-specific circuits in small numbers is not economic, because of the high costs associated with mask production.
Based on the above discussion, EBDW lithography may be a better candidate for low-cost electron beam lithography than electron beam projection lithography. EBDW does not use masks (i.e., is maskless), eliminating the mask costs and speeding up the semiconductor manufacturing process. EBDW lithography also has the potential to achieve improved resolution. Nevertheless, EBDW has a problem relating to its low throughput. For example, it may take about ten to one hundred hours to write a pattern over an entire wafer using EBDW lithography. One previous approach to attempt to increase the throughput is by increasing the beam current. However, when the current density exceeds a certain threshold, electron-electron interactions (e.g., repulsive Coulomb forces between electrons) cause the beam to blur and the spot size to increase. This limitation further limits throughput for existing electron beam lithography systems.
One solution to reduce electron-electron interactions is to spread the current over the photomask, the reticle, or the wafer using multiple electron beams that write simultaneously. Multi-beam writing improves the throughput of EBDW lithography by using a plurality of electron beams writing in parallel on the substrate instead of one single electron beam. This massive parallelism can circumvent the physical limitations of electron beam lithography systems, and can make EBDW appealing to cost and extendibility. Nonetheless, there are several challenges for multi-beam lithography. One of the challenges is to simultaneously control multiple electron beams in terms of individual beam placement, footprint, dose, and blur. It is even more challenging to achieve a compact multiple beam design by using the existing commercially available, bulky electron sources, such as thermionic emitters, which are usually made of tungsten or lanthanum hexaboride (LaB6), or Schottky emitters, which are typically made of a tungsten wire having a tip coated with a layer of zirconium oxide (ZrOx).
Electron beam lithography systems need an electron source to generate an electron beam directed towards a sample. Electron sources can be divided into two broad groups: thermionic sources and field emission sources. Thermionic sources are the most common commercially available electron emitters, and are usually made of tungsten or lanthanum hexaboride (LaB6). In thermionic emission, electrons are boiled off the material surface when the electron thermal energy is high enough to overcome the surface potential barrier. Even though thermionic emitters are widely in use, they typically require elevated temperatures (>1300 K) to operate, and may have several drawbacks such as inefficient power consumption, wide energy spread, short lifetime, low current density, and limited brightness. The demand for more efficient electron sources has driven the research and development of Schottky emitters and cold electron sources such as electron field emitters.
In the Schottky emitters, thermionic emission is enhanced by effective potential barrier lowering due to the image charge effect under an applied external electric field. Schottky emitters are typically made of a tungsten wire having a tip coated with a layer of zirconium oxide (ZrOx), which exhibits a much lower work function (˜2.9 eV). Schottky emitters are currently used in some electron beam systems. Despite being quite successful, thermally-assisted Schottky emitters still need to be operated at high temperature (>1000 K) and high vacuum (˜10−9 mbar), and have wider than desirable electron emission energy spread due to the high operating temperature. An electron source with lower energy spread, higher brightness (e.g., radiance) and higher current density than Schottky emitters may be desirable for semiconductor wafer and mask inspection, review, and lithography as it will enable faster and, hence, more cost effective, inspection, review, and lithography.
Cold electron sources, particularly electron field emitters are known in the art. Such emitters have been used in field emission displays, gas ionizers, x-ray sources, electron beam lithography, and electron microscopes, among other applications.
Field emission takes place when the applied electric field is high enough to reduce the potential barrier on the tip-vacuum interface so that electrons can tunnel through this barrier at a temperature close to room temperature (e.g., quantum-mechanical tunneling). A typical field-emitter comprises a conical emitter tip with a circular gate aperture. A potential difference is established across the emitter cathode, the gate and the anode under an applied external field, resulting in high electric field at the surface of the tip. Electrons tunnel through the narrow surface barrier and travel towards an anode, which is biased at a higher potential than the gate. The emission current density can be estimated by a modified version of the Fowler-Nordheim theory, which takes into account the field enhancement factor due to the field emitters.
Field emitters, because they can operate near room temperature, have lower energy spread than Schottky and thermionic emitters, and can have higher brightness and electron current than thermionic emitters. However, in practical use, the output current of a field emitter is less stable as contaminants can easily stick to the tip of the emitter and raise its work function, and hence lower the brightness and current. Periodic flashing (i.e., temporarily raising the tip temperature) is required to remove those contaminants. While the tip is being flashed, the instrument is not available for operation. In the semiconductor industry instruments are required to operate continuously and stably without interruption, so Schottky emitters are usually used in preference to cold field emitters.
Previous field emitter arrays (FEAs) had multiple conically shaped electron emitters arranged in a two-dimensional periodic array. These field emitter arrays can be broadly categorized by the material used for fabrication into two broad categories: metallic field emitters and semiconductor field emitters.
Early efforts have been concentrated on developing metallic field emitters. For example, Spindt-type molybdenum field emitters were developed because molybdenum has a low resistivity (53.4 nΩ·m at 20° C.) and a high melting point (2896 K). Nevertheless, metallic emitters suffer from several disadvantages such as lack of uniformity due to metal deposition techniques, and more severely the degradation in emission current, mainly due to oxidation.
With the advent of modern semiconductor fabrication technology, there has been investigation of semiconductor field emitters, particularly silicon field emitters.
Single-crystal (monocrystalline) silicon is an attractive material for field emitters. Silicon crystals can be grown with very high purity and very few crystal defects. The conductivity of silicon can be altered by doping and/or applying a voltage. More importantly, silicon has a well-developed technology base. The mature silicon integrated circuit (IC) technology makes it possible to fabricate arrays of silicon field emitters.
The structure of a typical prior-art silicon field emitter is shown in FIG. 8. A silicon substrate 61 is doped with impurities and can be either n-type or p-type doped. The cone-shaped emitter 64 is formed on the silicon substrate 61, with an optional gate layer 67 attached to a dielectric layer 66, which includes one or more insulating layers. The optional gate layer 67 controls and extracts the emission current. A third electrode, such as the anode (not shown), faces the gate layer 67 and is separated at a large distance (e.g., on the order of hundreds of microns) from the cathode. This is the typical silicon field emitter triode configuration. Note that without the gate layer 67, the field emitter can be used as a diode. Quantum tunneling of electrons takes place when a bias voltage is applied across the structure of the emitter 64. A large electrical field is generated on the surface of the tip of the emitter 64, and electrons are emitted from the tip.
Silicon field emitters are not yet commercially available. One serious problem with the use of silicon to form field emitters is that silicon is quite reactive, and can be contaminated within hours, even at pressures around 10−10 torr. Silicon very readily forms a native oxide on its surface. Even in a vacuum, a native oxide will eventually form as the small amounts of oxygen and water present in the vacuum will react with the surface of the silicon. The interface between silicon and silicon dioxide has defects (due to dangling bonds) where the probability of an electron recombination is high. Furthermore, the band gap of silicon dioxide is large (about 9 eV) creating an additional barrier higher than the work function that an electron has to overcome in order to escape, even if the oxide is very thin. For example, native oxide on a very smooth silicon surface is typically about 2 nm thick. In some circumstances, oxidation can also change the shape of the field emitters. These aforementioned problems may result in low brightness and current, unstable emission, low reliability, poor scalability and poor uniformity, which have hindered the commercial use of silicon field emitters.
Research effort has been expanded in looking for surface treatments and coatings for field emitters to improve their performance for lower turn-on voltages, higher emission current densities, lower noise, and improved stability. These treatments may include coating the emitter tips with refractory metals, silicides, carbides, and diamond. However, these coating materials are usually limited by the fabrication process in forming smooth and uniform coating surfaces, and/or are often affected by the oxide layer formed on the coating surfaces, creating an additional energy barrier. For these reasons, coated silicon field emitters have not become yet practical as cold electron sources.
Therefore, what is needed is an electron source that overcomes some, or all, of the limitations of the prior art. In addition, there is a need for a multiple electron beam lithography system with improved throughput that overcomes some, or all, of the deficiencies of previous systems.